Control several different materials via one slider
If using node materials you could use a node group:


Simply creating a the group containing just a RGB node.


Add > Input > RGB and Add > Group > Make Group.
or ShiftA, I, R (if it gives you Render Layer Node, then it's G), CtrlG.

Go to parent node tree (leave the new group)
For each material Add > Group > (Group Name) and use that as your color.


Edits to the group will effect all instances.